Fuel used in an internal combustion engine, in particular fossil fuel, contains trace amounts of sulfur (S) ingredients. The sulfur ingredients contained in fuel in this way invite deterioration of component parts in an exhaust system of an internal combustion engine etc. Further, if frequently performing control suppressing deterioration of the component parts due to the sulfur ingredients or control for regenerating deteriorated component parts, deterioration of the fuel efficiency is invited. Therefore, in order to keep the deterioration of the fuel efficiency etc. at a minimum extent while keeping the deterioration of the component parts at a minimum extent, it is desirable to detect the content of sulfur ingredients in the fuel.
If the fuel used in an internal combustion engine contains sulfur ingredients, the exhaust gas discharged from a combustion chamber contains sulfur oxide (SOX). Further, the higher the content of the sulfur ingredients in the fuel, the higher the concentration of SOX in the exhaust gas. Therefore, if it is possible to detect the concentration of SOX in exhaust gas, it is possible to estimate the content of sulfur ingredients in the fuel.